Introduction
The compound cyclosporine (cyclosporin A) has found wide use since its introduction in the fields of organ transplantation and immunomodulation, and has brought about a significant increase in the success rate for transplantation procedures. Recently, several classes of macrocyclic compounds having potent immunomodulatory activity have been discovered. Okuhara et al. disclose a number of macrocyclic compounds isolated from the genus Streptomyces, including the immunosuppressant FK-506, a 23-membered macrocyclic lactone, which was isolated from a strain of S. tsukubaensis (Okuhara et al., 1986).
Other related natural products, including FR-900520 and FR-900523, which differ from FK-506 in their alkyl substituent at C-21, have been isolated from S. hygroscopicus yakushimnaensis. Another analog, FR-900525, produced by S. tsukubaensis, differs from FK-506 in the replacement of a pipecolic acid moiety with a proline group. Unsatisfactory side-effects associated with cyclosporine and FK-506 including nephrotoxicity, have led to a continued search for immunosuppressant compounds having improved efficacy and safety, including an immunosuppressive agent which is effective topically, but ineffective systemically (Luly, 1995).
Rapamycin
Rapamycin is a macrocyclic triene antibiotic produced by Streptomyces hygroscopicus, which was found to have antifungal activity, particularly against Candida albicans, both in vitro and in vivo (Baker et al., 1978; Sehgal, 1975; Sehgal, 1976; Sehgal et al., 1975; Vezina et al., 1975).

Rapamycin alone (Surendra, 1989) or in combination with picibanil (Eng, 1983) has been shown to have anti-tumor activity. In 1977, rapamycin was also shown to be effective as an immunosuppressant in the experimental allergic encephalomyelitis model, a model for multiple sclerosis; in the adjuvant arthritis model, a model for rheumatoid arthritis; and was shown to effectively inhibit the formation of IgE-like antibodies (Martel et al., 1977).
The immunosuppressive effects of rapamycin have also been disclosed in FASEB, 1989, 3, 3411 as has its ability to prolong survival time of organ grafts in histo-incompatible rodents (Morris and Meiser, 1989). The ability of rapamycin to inhibit T-cell activation was disclosed by M. Strauch (FASEB, 1989, 3, 3411). These and other biological effects of rapamycin have been previously reviewed (Morris, 1992).
Rapamycin has been shown to reduce neointimal proliferation in animal models, and to reduce the rate of restenosis in humans. Evidence has been published showing that rapamycin also exhibits an anti-inflammatory effect, a characteristic which supported its selection as an agent for the treatment of rheumatoid arthritis. Because both cell proliferation and inflammation are thought to be causative factors in the formation of restenotic lesions after balloon angioplasty and stent placement, rapamycin and analogs thereof have been proposed for the prevention of restenosis.
Mono-ester and di-ester derivatives of rapamycin (esterification at positions 31 and 42) have been shown to be useful as antifungal agents (Rakhit, 1982) and as water soluble prodrugs of rapamycin (Stella, 1987).
Fermentation and purification of rapamycin and 30-demethoxy rapamycin have been described in the literature (Paiva et al., 1991; Sehgal et al., 1983; Sehgal et al., 1975; Vezina et al., 1975).
Numerous chemical modifications of rapamycin have been attempted. These include the preparation of mono- and di-ester derivatives of rapamycin (Caufield, 1992), 27-oximes of rapamycin (Failli, 1992a); 42-oxo analog of rapamycin (Caufield, 1991); bicyclic rapamycins (Kao, 1992a); rapamycin dimers (Kao, 1992b); silyl ethers of rapamycin (Failli, 1992b); and arytsulfonates and sulfamates (Failli, 1993). Rapamycin was recently synthesized in its naturally occurring enantiomeric form (Hayward et al., 1993; Nicolaou et al., 1993; Romo et al., 1993).
It has been known that rapamycin, like FK-506, binds to FKBP-12 (Bierer et al., 1991; Dumont et al., 1990; Fretz et al., 1991; Harding et al., 1989; Siekierka et al., 1989). Recently it has been discovered that the rapamycin/FKBP-12 complex binds to yet another protein, which is distinct from calcineurin, the protein that the FK-506/FKBP-12 complex inhibits (Brown et al., 1994; Sabatini et al., 1994).
Other drugs have been used to counter unwanted cell proliferation. Exemplary of these is paclitaxel. A complex alkaloid extracted from the Pacific Yew, Taxus brevifolia, paclitaxel stabilizes components of the cell skeleton (tubulin, the building blocks of microtubules) that are critical in cell division, thus preventing cell proliferation (Miller and Ojima, 2001).
Stents
Percutaneous transluminal coronary angioplasty (PTCA) was developed by Andreas Gruentzig in the 1970's. The first canine coronary dilation was performed on Sep. 24, 1975; studies showing the use of PTCA were presented at the annual meetings of the American Heart Association the following year. Shortly thereafter, the first human patient was studied in Zurich, Switzerland, followed by the first American human patients in San Francisco and New York. While this procedure changed the practice of interventional cardiology with respect to treatment of patients with obstructive coronary artery disease, the procedure did not provide long-term solutions. Patients received only temporary abatement of the chest pain associated with vascular occlusion; repeat procedures were often necessary. It was determined that the existence of restenotic lesions severely limited the usefulness of the new procedure. In the late 1980's, stents were introduced to maintain vessel patency after angioplasty. Stenting is involved in 90% of angioplasty performed today. Before the introduction of stents, the rate of restenosis ranged from 30% to 50% of the patients who were treated with balloon angioplasty. The recurrence rate after dilatation of in-stent restenosis may be as high as 70% in selected patient subsets, while the angiographic restenosis rate in de novo stent placement is about 20%. Placement of the stent reduced the restenosis rate to 15% to 20%. This percentage likely represents the best results obtainable with purely mechanical stenting. The restenosis lesion is caused primarily by neointimal hyperplasia, which is distinctly different from atherosclerotic disease both in time-course and in histopathologic appearance. Restenosis is a healing process of damaged coronary arterial walls, with neointimal tissue impinging significantly on the vessel lumen. Vascular brachytherapy appears to be efficacious against in-stent restenosis lesions. Radiation, however, has limitations of practicality and expense, and lingering questions about safety and durability.
Stents and Combination Therapies
The major effort undertaken by the interventional device community to fabricate and evaluate drug eluting stents has met the original goal by reducing restenosis by at least 50%. However, there still remains a need for improved local drug delivery devices, e.g., drug-impregnated polymer-coated stents, that provide safe and efficacious tools for preventing and treating restenosis. For example, the two commercially available single-drug elution stents reduce restenosis and improve patient outcomes, but do not eliminate restenosis or are free of adverse safety issues. Patients, and especially at-risk patients, including diabetics, those with small vessels and those with acute coronary syndromes, could benefit from local drug delivery devices, including stents with improved capabilities. Drug delivery devices including combinations of drugs are known. However, the art does not appear to teach particularly effective drug combinations administered locally, e.g., eluted from a stent. For example, and as discussed more below, Falotico teaches an EVA-PBMA polymer-coated stent including a rapamycin/dexamethasone combination that was “far less effective” in reducing neointimal area, percent-area stenosis, and inflammation scores than stents delivering either rapamycin alone or dexamethasone alone (Falotico, 2003).